1. Field of the Invention
This invention relates generally to improved oxidant feed plenums and power lead cooling for tubular solid oxide fuel cells (SOFCs) disposed in a fuel cell generator.
2. Background Information
High temperature, solid oxide electrolyte fuel cell generators, which are made of mostly ceramic components, including supported tubular fuel cells and oxidant/air feed tube conduits, and which allow controlled leakage among plural chambers in a sealed housing, are well known in the art, and are taught, for example, in U.S. Pat. No. 5,573,867 (Zafred, et al., which taught recirculation of spent fuel through a recirculation channel to mix with feed fuel at an internal ejector/pre-reformer). Oxidant/air feed tube conduit support systems were taught in U.S. Pat. Nos. 4,664,986, and 5,733,675 (Draper, et al., and Dederer, et al., respectively) and also in U.S. Pat. Nos. 4,808,491 and 4,876,163 (both Reichner). The prior art system of Draper et al. taught welding metal air feed tube conduits to associated metal subheader plenums, providing a rigid, metal feed tube support system.
In the prior art Reichner system, as shown in FIG. 1 of the present application (and as generally shown in FIG. 1A of the Reichner ""491 patent), oxidant/air feed 50 flowed into top metal oxidant/air distribution plenum 52 and then into further oxidant/air distribution plenums 52xe2x80x2, where the oxidant/air then passed downward into fuel cells via individual ceramic oxidant feed tubes 51. At the top of the oxidant feed tubes 51, spherical supports 70 kept the oxidant feed tubes in place. These spherical supports required a machined spherical seat 72 in the Inconel plenum wall 74 at the bottom of the plenums 52xe2x80x2. Insulation 76, in a brick like configuration, surrounded the plenums. Steel outer generator enclosure 85 surrounded the fuel cell generator. Exhaust gas passages are shown as channels 80 and the bottom lower plenum enclosure insulation board is shown as 82, supporting the bottom of metal plenum 52xe2x80x2. Also shown are tubular fuel cells 36, metal wool interconnection material 34, which was attached to the top, bottom and sides of the fuel cells and which connected to vertical internal metal D.C. power leads 32 through metal cables 34 and series vertical metal connection plate 17. Inner metal canister 6 and pre-heating combustion chamber 94 are also shown. Feed fuel 12 passed upwards along the outside of the fuel cells 36, with part of the spent fuel 14 being recirculated and part of the spent fuel 16 being passed into combustion chamber 94.
The Draper et al. feed tube support system design was very expensive, very heavy, and required major machining and welding of Inconel components. The Reichner design also required substantial machining to properly set the spherical support and the D.C. power leads 32 required active cooling to dissipate heat as a result of high ambient temperature and ohmic losses associated with the internal metal wool interconnections shown as 34. As the number of fuel cells increase, so the voltage at the module terminals would require complex cooling of all the internal power leads. U.S. Pat. No. 4,431,715 (Isenberg) solved many power lead problems but not the cooling problem associated with large SOFC generators.
Internally, the SOFC generator module of the present 100 kw class design includes a plurality of metallic air manifolds, located right above the fuel cell bundles/stacks, which are designed to uniformly distribute process air to each of the fuel cells within the stack. As the number of stacks is increased to produce more power, so is the number of metal manifolds required which must be branched to larger upper metal manifolds to provide equal flow distribution. As an example, a 1 MW generator module with five 100 kW size stacks of the present design would require 40 small metallic air manifolds, coupled to 10 intermediate metal manifolds which must be then connected to 2 or more larger metal air plenums. This proliferation of metal manifolds and metal branch systems results in high pressure losses, difficult high temperature sealing problems, complex support structures to support the heavy metal manifolds and overall high manufacturing costs. Additionally, the use of cut brick type blocks of ceramic insulation, shown as 76 in FIG. 1, while helping to provide support for the fuel cell stacks during shipping adds significantly to overall cost of the generator.
Another problem with current SOFC systems is the external ducting arrangements required to couple steamers, recuperators, preheaters and the like which reduces overall efficiency and causes substantial heat losses. Although U.S. Pat. No. 5,741,605 (Gillett, et al.) introduced modular concepts including a pre-assembled, self-supporting removable modular fuel cell stack, such major components as turbine/generators, compressors and recuperators, were separated from the fuel cells and still required substantial ducting.
What is needed is an improved, simpler, less expensive oxidant/air feed tube and support system that will require no metal finishing, a power lead design that minimizes cooling requirements, and better utilization of the interior insulation. It would also be desirable to eliminate external ducting to auxilaries such as blowers, air preheaters and recuperators. Therefore, it is one of the main objects of this invention to provide a simpler, significantly less expensive oxidant/air feed tube support system which requires minimal or no metal finishing. It is also a main object of this invention to provide power lead designs requiring less cooling and a new insulation design.
These and other objects of the invention are accomplished by providing a solid oxide fuel cell generator characterized in that it comprises: (1) stacks of hollow, tubular axially elongated fuel cells having an open top and closed bottom, with interior air electrodes and exterior fuel electrodes with solid electrolyte therebetween, which can operate on feed oxidant and feed fuel to generate electricity; (2) a single oxidant inlet plenum formed by enclosing insulation, including a bottom enclosing member having holes therethrough constituting an oxidant feed tube positioning board, located at the top portion of the fuel cell generator; (3) fuel inlet plenum, located at the bottom portion of the fuel cell generator; (4) reacted oxidant/fuel exhaust chamber, located above the fuel cells and below the oxidant inlet plenum; (5) power leads electrically connected to the fuel cells transverse to the axis of the fuel cells; and (6) a plurality of low-cost oxidant feed tubes supported by the oxidant feed tube positioning board and passing through the reacted oxidant/fuel exhaust chamber into the center of the fuel cells; all surrounded by insulation; and all within an outer generator enclosure; where there are at least two fuel cell stacks arranged in a row next to each other, the oxidant feed tube positioning board at the bottom of the oxidant inlet plenum is a composite sandwich of thin woven ceramic fiber sheets impregnated with ceramic adhesive bonded to a thick porous core of alumino-silicate ceramic fibers bonded with an inorganic binder, wherein the insulation constitutes, primarily, bulk ceramic fibers. The core of the oxidant feed tube positioning board is preferably a vacuum formed alumino-silicate fiber board. When laminated with a ceramic, woven sheet on both faces, it is an extremely stiff, lightweight structure with substantial strength and low gas permeability. The ceramic fibers used for about 70% to 80% of the insulation throughout the fuel cell generator are preferably bulk alumino-silicate uniformly packed at a density between about 128 to 160 kg/cubic meter (8 to 10 pounds/cubic foot). Additionally, the external ducting is kept to a minimum to prevent heat losses, by integral entrance and exit conduits where, for example, an oxidant feed pre-heater could be easily bolted onto the oxidant inlet plenum entrance conduit. The top of the generator is designed to have a top lid comprising a metal cover and attached insulation allowing ease of access to the feed tubes and fuel cells.